The invention relates to a vehicle having a multiphase electric machine, comprising a first onboard power subsystem provided with a first nominal DC voltage level, and a second onboard power subsystem provided with a second nominal DC voltage level, wherein the electric machine comprises a rotor, a first stator system, and a second stator system. The first onboard power subsystem comprises a first inverter having a first intermediate circuit capacitor. The first stator system is dedicated to the first inverter. The second onboard power subsystem comprises a second inverter having a second intermediate circuit capacitor, and the second stator system is dedicated to the second inverter.
Typically, components in a vehicle, which are electrical energy consumers, are supplied by an onboard power supply network with a nominal DC voltage level of 14 volts. A secondary 12 volt energy store, which in the onboard power system assumes the function of an energy source or the function of an energy sink, depending on the operating state, and a 14 volt generator are designed to provide an electrical power output of 2-3 kW in the vehicle.
If several consumers with higher power requirements are integrated in the onboard power system, the onboard power system can be provided with two onboard power subsystems. A DC regulator then transfers electrical power between the two onboard power subsystems. The electric machine, which in a vehicle having an electrified drive train can also be motor-operated, has, apart from at least one energy store per onboard power subsystems, the function as electrical energy source or energy sink in the vehicle. An onboard power system topology such as this is illustrated in the patent specification of DE 102 44 229 A1, for example.
It is an object of the invention to describe an improved vehicle having an electric machine and two onboard power subsystems, and a method for operating said electric machine.
This objective is attained by a vehicle according to claim 1. Advantageous embodiments and further developments of the invention are the subject matter of the dependent claims.
According to the invention, both the first stator system and the second stator system are configured in a star connection, and a transfer circuit connects the star point of the first stator system to the star point of the second stator system. This means that the two star points of the stator systems can be directly coupled to one another electrically.
According to a preferred embodiment of the invention, the transfer circuit comprises a first diode and a second diode, which are connected in opposite directions and in series.
The transfer circuit further comprises a first switch connected in parallel to the first diode, or as an alternative, a second switch connected in parallel to the first diode.
It is also particularly beneficial if the transfer circuit comprises the first switch that is connected in parallel to the first diode, and the second switch that is connected in parallel to the second diode.
The diodes connected in opposite directions ensure that, with the first switch open and/or the second switch open, the direct electrical coupling of the two star points is ineffective. With the first switch closed and/or with the second switch closed, there is a direct electrical connection between the two star points in form of a very low ohmic connection by way of a series connection of two closed switches, or a series connection of a switch and a diode.
In a further variation of the invention, the first inverter is provided with three high side switches and three low side switches, and the second inverter is provided with three high side switches and three low side switches. The three high side switches of the first inverter and the three low side switches of the first inverter can be actuated by pulse-width modulation. The three high side switches of the second inverter and the three low side switches of the second inverter can be actuated by pulse-width modulation, with the first switch open and with the second switch open, the electric machine can be motor-operated or generator-operated, or in a mixed mode, by pulse-width modulated actuation of the high side switches and the low side switches of the first inverter and the second inverter. Connected in parallel to the low side switches and to the high side switches is a low side diode, or a high side diode, respectively.
This means that with respect to both onboard power systems, the machine can be used as a generator or as a motor, regardless whether the machine of the respective other onboard power system at a given time of operation is being utilized as a motor or as a generator. When operated as a generator, the respective onboard power system is supplied with electrical energy via the respective stator system as a result of a torque impressed on the rotor externally (for example, by a combustion engine of the vehicle). When operated as a motor, electrical energy is extracted from the respective onboard power system via the respective stator system and is converted to rotational energy of the rotor, which is tapped externally (for example, by a belt-driven consumer) from the rotor as torque.
It is particularly beneficial if the first nominal DC voltage level exceeds the second nominal DC voltage level in the direction of higher voltage level with respect to a reference voltage in the vehicle, for example, an electrical mass of the vehicle shared by the two onboard power subsystems, and when the rotor is stationary, the electric machine can be operated as DC voltage step-down converter between the first onboard power subsystem and the second onboard power subsystem.
The electric machine can be operated as DC voltage step-down converter because in a stationary mode, that is, as soon as a continuous current flow from one onboard power subsystem to the other onboard power subsystem is established, the low side switches of the second inverter are opened, the high side switches of the second inverter are closed, the low side switches of the first inverter are opened, and the high side switches of the first inverter are controlled in a pulse-width modulated manner.
As an alternative, the low side switches of the first inverter can also be controlled in a pulse-width modulated manner complementary to the high side switches of the first inverter, in order to reduce transmission losses.
In this regard, in order to prevent a bridge short-circuit between high and low side switches, a dead time is to be provided, during which both high side and low sides switches are open.
In addition, the excitation winding of the rotor can be electrically short-circuited.
When the high side switches of the first inverter are closed, the voltage difference between the first onboard power subsystem and the second onboard power subsystem lies across the effective inductance, which is formed by the three parallel-connected inductances of the first stator system in series to the three parallel-connected inductances of the second stator system. During this switch-on time, the current in the inductance increases linearly, and the average value thereof can be tapped off a load as direct current. During the switch-off phase, the inductance decreases the energy content while the intermediate circuit capacitor of the second onboard power subsystem is being charged. In order to form a freewheel for the current, the low side switches of the first inverter can either be closed or can remain open. In the latter case, the low side diodes of the first inverter lead.
In order to improve the start-up behavior of the DC-to-DC operation, that is during the transition from a rotating operation to a standstill operation, the low side switches of the second inverter can initially be closed, and the high side switches of the second inverter can be opened. Thus, the full voltage of the first onboard power subsystem lies across the effective inductance instead of the difference of the voltage between the first onboard power subsystem and the second onboard power subsystem, by way of which a higher power increase can be achieved. The transition into the stationary mode is controlled by pulse-width modulated operation of the high side switches and the low side switches of the second inverter.
Furthermore, it is particularly advantageous if the first nominal voltage level exceeds the second nominal voltage level towards higher nominal voltage level, and with the rotor being stationary, the electric machine can be operated as a DC step-up converter from the second onboard power subsystem to the first onboard power subsystem.
The electric machine can be operated as a DC step-up converter because the low side switches of the second inverter are opened, the high side switches of the second inverter are closed, the high side switches of the first inverter are opened, and the low side switches of the first inverter are controlled in a pulse-width modulated manner.
In addition, the excitation winding of the rotor can be electrically short-circuited.
In order to reduce transmission losses, the high side switches of the first inverter can also be controlled in a pulse-width modulated manner complementary to the low side switches of the first inverter. In this case, in order to prevent a bridge short-circuit between high and low side switches, a dead time is to be provided, during which high side and low sides switches are open.
If the low side switches of the first inverter are closed, the voltage of the second onboard power subsystem lies across the effective inductance, which is formed by the three parallel-connected inductances of the first stator system in series to the three parallel-connected inductances of the second stator system. During this switch-on time, the current in the inductance increases linearly, and the induction builds up energy content. Simultaneously, the high side diodes of the first inverter block so that the voltage at the intermediate circuit capacitor of the first onboard power subsystem can be adjusted to the voltage of the second onboard power subsystem. During the switch-off phase, the induction decreases the energy content, and the intermediate circuit capacitor of the first onboard power subsystem is being charged. The high side switches of the first inverter can thereby either switched on or remain closed. In the latter case, the high side diodes of the first inverter lead.
Hereafter, a preferred exemplary embodiment of the invention is described with reference to the enclosed drawings. Therefrom, further details, preferred embodiments, and further developments of the invention become evident, wherein is shown schematically in:
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.
Identical reference numerals specify identical technical characteristics.